Multi-ply absorbent sheet such as towel and tissue sometimes conventionally include a plurality of glue-bonded layers or plies. Adhesives employed include aqueous polyvinyl alcohol (PVOH) solutions as is disclosed in U.S. Pat. No. 3,414,459. Such solutions are relatively low solids, less than 10% by weight PVOH and exhibit Newtonian Fluid viscosity characteristics; that is, wherein viscosity is substantially independent of shear. Tissue/towel ply bonding is typically carried out in the following steps: 1) embossing one or more plies; 2) applying glue to the web on the raised emboss pattern elements; 3) bringing one or more plies into contact with the glued surface and applying sufficient pressure to the mated web to provide enduring plybond once the glue has dried. This process places several demands on the glue. It must penetrate the first web, but not too much. Enough tackiness must remain to stick to the non-glued web. The remaining glue must preferably penetrate the nonglued web to improve the integrity of the bond. See, also, U.S. Pat. No. 5,858,554 to Neal et al., entitled Paper Product Comprising Adhesively Joined Plies which describes ply-bonded absorbent sheet provided with polyvinyl alcohol or starch adhesive compositions, note Col. 4, lines 20-55.
A paper/glue bond can be considered a fiber/polymer composite structure with a tendency to fracture at the interface of the plies. The process and product requirements present a difficult balancing act. If the glue is too viscous, it may not penetrate either ply sufficiently. If the glue is too thin, excess glue is required to overcome the excess wicking of glue into the web. Sometimes increased marrying roll pressure is used in an attempt to overcome the performance of the glue; however, bulk, softness and emboss quality suffer when too much pressure is used.
There is a need for a better ply bonding adhesive that overcomes the mentioned deficiencies.
Nanofibrillated cellulose (NFC) or sometimes referred to as microfibrillated cellulose (MFC) is known in the art to be useful for a variety of purposes, including for use as a structural material in sheet and related articles. For example, in U.S. Pat. No. 6,734,335 it is mentioned that NFC is useful for use in absorbent structures. Col. 22, lines 13+. See, also, U.S. Pat. No. 7,614,110, Col. 13, lines 38+. United States Patent Application Publication No. US 2012/0219816 discloses use of NFC as a layer in a multilayer paperboard structure, Abstract. See, generally, United States Patent Application Publication No. US 2012/0058536, ¶ [0151], which discloses NFC as a structural material. NFC is used in molded structures, as seen in United States Patent Application Publication No. US 2009/0308552, ¶ [0001], as well as United States Patent Application Publication No. US 2011/0263756, Abstract. NFC is, likewise, known for use in adhesives. JP 60250079 discloses a liquid adhesive made by blending a polyvinyl acetate emulsion, sodium carboxymethyl cellulose and above 3-4% NFC based on the weight of the liquid composition. See, also, United States Patent Application Publication No. US 2010/0285295, ¶ [0023], where NFC is mentioned as a filler for an adhesive resin; United States Patent Application Publication No. US 2011/0052881, ¶ [0062], having similar discussion, as well as United States Patent Application Publication No. US 2009/0042003, ¶ [0057].